The EXISTS condition in SQL is used to check whether the result of a correlated nested query is empty (contains no tuples) or not. The result of EXISTS is a boolean value True or False. It can be used in a SELECT, UPDATE, INSERT or DELETE statement.

Syntax:

SELECT column_name(s) 
FROM table_name
WHERE EXISTS 
  (SELECT column_name(s) 
   FROM table_name
   WHERE condition);

Examples:
Consider the following two relation “Customers” and “Orders”.

Queries

1. Using EXISTS condition with SELECT statement
   To fetch the first and last name of the customers who placed at least one order.

   SELECT fname, lname 
   FROM Customers 
   WHERE EXISTS (SELECT * 
                 FROM Orders 
                 WHERE Customers.customer_id = Orders.c_id);

   Output:
   

2. Using NOT with EXISTS
   Fetch last and first name of the customers who has not placed any order.

   SELECT lname, fname
   FROM Customer
   WHERE NOT EXISTS (SELECT * 
                     FROM Orders 
                     WHERE Customers.customer_id = Orders.c_id);

   Output:
   

3. Using EXISTS condition with DELETE statement
   Delete the record of all the customer from Order Table whose last name is ‘Mehra’.

   DELETE 
   FROM Orders
   WHERE EXISTS (SELECT *
                 FROM customers
                 WHERE Customers.customer_id = Orders.cid
                 AND Customers.lname = 'Mehra');

   SELECT * FROM Orders;

   Output:
   

4. Using EXISTS condition with UPDATE statement
   Update the lname as ‘Kumari’ of customer in Customer Table whose customer_id is 401.

   UPDATE Customers
   SET lname = 'Kumari'
   WHERE EXISTS (SELECT *
                 FROM Customers
                 WHERE customer_id = 401);

   SELECT * FROM Customers;

   Output: